The invention relates to a photoelectric position measuring apparatus and in particular to a position measuring apparatus that utilizes interference by partial light beam bundles of separate orders of diffraction.
A prior position measuring apparatus is disclosed in U.S. Pat. No. 4,792,678 which is hereby incorporated herein by reference. One of the embodiments of the '678 patent shows two linear divisions for the deflection of the positive and negative diffraction beams (e.g., FIG. 3 of the '678 patent). Measuring signals are obtained from interferences of the positive and negative diffraction beams of one order (e.g., +1, -1).
The dissertation "Dreigitterschrittgeber" by J. Willhelm, 1978, Technische Universitat Hannover, page 52, describes a "four-grid generator." As described, this device has high sensitivity with reference to the grid constant. The device described in that publication includes a 120.degree. prism that serves for the reflection of partial beam bundles. The publication also states that it is advantageous that entry and exit beam areas are separate, so that inner reflections do not have an interfering effect. However, with such arrangement the Moire sensitivity can be disadvantageous. Interferences can arise which are due to the sensitivity of the position measuring apparatus due to turnings of the grids to one another.
Accordingly, it is an object of the present invention to provide a high-resolution, relatively simply constructed position measuring apparatus in which the evaluation of interfering beam bundles leads to phase-shifted signals with a high degree of modulation.
It is a further object of the present invention to provide a position measuring apparatus which is insensitive to turnings or tiltings of grids to one another.